Another thing that’s important to note is that money supply alone doesn’t determine interest rates in an unregulated economy.
Try this. Draw some first quadrant axis and label the X axis “Q” for Quantity, and the Y axis “i” for Nominal Interest Rate.
Draw a diagonal line going from the bottom left to the top right and label this “M” for Money Supply. As (i) increases, banks will be more likely to offer riskier loans, because the return will be higher for them. Therefore, the supply of money will increase as interest rates rise.
Draw a diagonal line going from the top left to the bottom right. Label this one “D” for Demand. As (i) increases, consumers will demand less money from banks, because their return will be lower. Therefore the demand for money will decrease as interest rates rise.
The point where the demand for money equals the supply of money is the point of equilibrium.
Now your question is how the interest rate changes with the supply of money held constant. In an unregulated economy, the interest rate is determined by the equilibrium point of this graph. If we hold (M) constant and shift (D) then we can change the interest rate. How does this happen?
Well, for one example let’s look at Price Level (or “P” for short). If (P) rises, and (i) stays the same, then “r,” the Real Interest Rate, is going to drop. This is because it will cost less, in terms of real value, to take out a loan at the same interest rate. Therefore, if (P) rises, then the entire (D) curve is going to shift right, because at every given (i), people will demand more money. The inverse is also true.
When (D) shifts right, and (M) stays the same, the point of equilibrium moves to a higher (i) and (Q). This means that if people demand more money, then more money will be put into the economy, but the price of that money is going to rise in the form of interest rates. There are many other things that can affect the relationship between (D) and (M), but I think you can probably see the basics at this point. Hope that helps!